A centrifuge of this general type is known from U.S. Pat. No. 5,505,684 in which a rotatable motor part is directly fixed to the rotor instead of being attached to it by means of an intermediate shaft as in older designs. As a result, high compactness is achieved, which is especially appropriate for small laboratory centrifuges.
In the known device, the static part of the electric motor is mounted radially inside the rotor. The consequence is a conventional motor with a cylindrical gap concentric with the axis of rotation, however in this instance entailing substantial drawbacks.
If for high motor efficiency the gap is made narrow, then rotor vibrations caused by imbalances relative to the stator must be suppressed. Therefore the known design provides a bearing rigidly joined to the stator. All the generated vibrations are transferred to the stator. This design offers no way to spring-dampen the vibrations inside the centrifuge.
Moreover the entire electric motor is concentrically enclosed by the rotor. Heat problems arise on that account. Because of long and high motor loads, the heat generated inside the rotor essentially can only be dissipated through the rotor. Test samples present inside the rotor will be heated: this feature in general is unacceptable.
The conventional motor design with a cylindrical gap moreover constrains the electric motor within the available space inside the rotor: for instance, the coils must be wound in a complex manner onto the pole pieces. In addition, the radially inward space available for the pole pieces and the coils decreases rapidly and considerable problems of mounting space and size arise, entailing heat problems and limits on motor output. Therefore only those electric motor designs are applicable that offer high efficiency, but these are also complex and costly.